Epidemiological and experimental studies indicate that estrogen treatment reduces cardiovascular disease in women and female experimental animals. Estrogen directly affects functions of endothelial and smooth muscle cells. Estrogen could also influence development of cardiovascular disease through modulating functions of blood elements. Effects of estrogen on platelets are little known and controversial. Understanding how estrogen therapy modulates platelets is important as platelets are physiological "first-responders" at the site of vascular injury be it mechanical denudation or endothelial dysfunction. Since platelets release both vasoactive and mitogenic factors, they are key in establishing conditions for immediate vasoconstriction and long-term remodeling of the vascular wall. The central hypothesis of this renewal application is that estrogen replacement REDUCES vascular response to injury by reducing platelet activation, release of platelet- derived vasoactive and mitogenic factors and responses of endothelium and vascular smooth muscle to platelet-derived factors. These actions of estrogen require both genomic and non-genomic receptor-mediated mechanisms. A unique approach will be taken to examine effects of estrogen on platelet count, irritability, contents and turnover in vitro and in vivo. Relationship between platelet functions and expression of estrogen receptors will be identified in estrogen-treated animals and mice lacking estrogen receptors (estrogen receptor knockout mice). Production of endothelium-derived nitric oxide and endothelin-l will be examined in response to platelet-derived factors from estrogen-treated animals. In addition, regulation of intracellular calcium, contraction and proliferation of the smooth muscle to platelet- factors will be examined. As thrombosis is a major side-effect in women taking estrogen replacement therapy including the new selective estrogen receptor modulators (SERMS), systematic studies are needed in order to better understand specific effects of estrogens on platelet function and interaction of platelets with the vascular wall. The experiments of this proposal take such an approach using molecular to integrated whole animal physiology.